This invention is generally directed to the processing of video signals associated with video imaging devices. It is more particularly directed to a method and apparatus for generating electrical signals representative of the blue components of light images received by the imaging device of a color television camera.
In the past, it has been proposed that an imaging system for a color television camera include a solid state image pick-up device which vertically and horizontally divided into light-sensitive areas or picture elements. Typically, incident light causes the generation of free electrons within the picture elements, the number of electrons generated being proportional to the intensity of the incident light. The free electrons, in turn, are used to generate output signals representative of the intensity of incident light.
In order to generate signals representative of the color of the incident light, a filter may be interposed between the lens system of the camera and the solid state pickup device. Such a filter is normally divided into filter stripes corresponding in number to the vertical areas in the pick-up device, one filter stripe overlying each vertical area or set of vertical areas in the pick-up device. Further, each filter stripe is selected to pass light of predetermined wavelengths and to absorb or reflect light of differing wavelengths. For example, such a filter might have successive, repeating sets of vertical filter stripes denoted as white (W), minus red (-R), minus blue (-B) and green (G). White light (a combination of red, blue and green light) passes through the white filter stripe, blue and green light pass through the minus red filter stripe, green and red light pass through the minus blue filter stripe, and green light (corresponding to minus red and minus blue) passes through the green filter stripe. Thus, each vertical area on the pick-up device receives only the light of the color or colors transmitted by its corresponding filter stripe. Herein, the light-sensitive picture elements on the pick-up device are referred to by the color of light incident upon them, i.e., W, -R, -B, or G.
The pick-up device can be thought of as being divided into repeating, horizontally spaced sets of picture elements, each set having four elements, for example, a W, a -R, a -B and a G element. To generate a signal corresponding to the light incident upon one set of light-sensitive elements, the outputs of each element in the set is sampled and decoded to generate an electrical signal indicative of the average red, blue and green components of the light incident on the entire sampled set of elements.
To generate a complete raster, successive sets of elements are scanned and sampled to generate a composite electrical signal corresponding to the light reflected to the pick-up device from an object or scene.
One reason why it has been thought necessary to include four light-sensitive elements in each sampled set of elements is that blue light, typically of a lower luminous intensity than red or green light, requires a greater sampling area to generate a signal having a desirable signal-to-noise ratio. Signals corresponding to red and green light can be sampled by using only a selected two or one, respectively, of the light-sensitive elements in each sampled set. Accordingly, all light which is received by one set of elements is sampled and decoded to generate an electrical signal indicative of the average blue light incident on the sampled set, even though any one or more of the light-sensitive elements may receive no blue light. This aspect of prior filter systems leads to undesirable results.
For example, should a color transition or a change in the intensity of a color occur within the area corresponding to a sampled set of light-sensitive elements, the blue component of the decoded signal may be of an incorrect amplitude. A further undesirable aspect of such prior systems is that, although blue light may be incident upon fewer than four of the light-sensitive elements in a sampled set, i.e., a color transition occurs within the sampled set, the decoded signal representative of blue light incident on that sampled set will incorrectly indicate that blue light is incident on each light-sensitive element of the sampled set. Moreover, such a decoded signal may also incorrectly indicate where the color transition occurs. The latter problem usually shows up as a trailing blue edge which occurs after the actual color transition and appears to a viewer as a form of misregistration.
The above-described problems are associated primarily with the generation of signals corresponding to the blue components of incident light. The signals representative of red and green light do not share such problems to as great an extent.
Accordingly, it is an object of this invention to provide a method and apparatus for generating an electrical signal representative of the blue component of light incident upon a light-sensitive imaging device which substantially overcomes the above-described problems.